It works on all your companions, not just the ones in your current party.

It may be preferable to save this item for as long as possible; since each level requires 1000 more experience than the last, you may be able to boost yourself to an even higher level than you would otherwise using it at a lower level.

On Nightmare difficulty, it may be advisable to avoid using this potion altogether; after update 1.03, enemies scale in power and health to Hawke's level, and by the time you can craft it you've already completed the characters' core builds. An additional talent point and 3 extra attribute points may not be worth elevating enemies across the board, while also de-valuing your equipment by another star (indicating reduced effectiveness relative to enemy strength).